Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
Description of the Related Art
Conventionally, there are known to be image forming apparatuses that form electrostatic latent images on a photosensitive member by using a rotating polygonal mirror to deflect a light beam emitted from a light source and scanning the photosensitive member with the deflected light beam. This kind of image forming apparatus includes an optical sensor (beam detection (BD) sensor) for detecting the light beam deflected by the rotating polygonal mirror, and the optical sensor generates a synchronization signal upon detecting the light beam. By causing the light beam to be emitted from the light source at a timing determined using the synchronization signal generated by the optical sensor as a reference, the image forming apparatus aligns the writing start positions for the electrostatic latent image (image) in the direction (main scanning direction) in which the light beam scans the photosensitive member.
Also, there are known to be image forming apparatuses that include multiple light-emitting portions (light emitting elements) as a light source for emitting multiple light beams that each scan different lines on the photosensitive member in parallel in order to realize a higher image formation speed and higher resolution images. With this kind of multi-beam image forming apparatus, a higher image formation speed is realized by scanning multiple lines in parallel using multiple light beams, and higher resolution images are realized by adjusting the interval between the lines in the sub-scanning direction.
Japanese Patent Laid-Open No. 2008-89695 discloses an image forming apparatus that includes multiple light-emitting portions (light emitting elements) as a light source and is capable of adjusting the resolution in the sub-scanning direction by performing rotational adjustment of the light source in the plane in which the light-emitting portions are arranged. This kind of resolution adjustment is performed in the step of assembling the image forming apparatus. Japanese Patent Laid-Open No. 2008-89695 discloses a technique for suppressing misalignment in the writing start positions in the main scanning direction for the electrostatic latent image that occurs due to light source attachment errors in the assembly step. Specifically, the image forming apparatus uses a BD sensor to detect light beams emitted from a first light-emitting portion and a second light-emitting portion and generates multiple BD signals. Furthermore, the image forming apparatus sets a light beam emission time for the second light-emitting portion relative to the light beam emission time for the first light-emitting portion based on the difference in the generation times of the generated BD signals. This compensates for light source attachment errors in the assembly step and suppresses misalignment in the writing start positions for the electrostatic latent image between the light-emitting portions.
Also, with an image forming apparatus that includes multiple light-emitting portions (light emitting elements) as a light source, such as that described above, there are cases where the light-emitting portions are driven by one laser driver IC, and there are cases where the light-emitting portions are driven by multiple laser driver ICs. For example, Japanese Patent Laid-Open No. 2011-173412 proposes a method in which control states can be mutually monitored between multiple laser driver ICs, and the timing at which the laser driver ICs execute APC is controlled based on the monitoring result.
As described above, with an image forming apparatus that includes multiple light-emitting portions as a light source, in the case where the difference in the generation times of two BD signals generated by a BD sensor (BD signal time interval) is to be measured, the light power of the light beams incident on the BD sensor needs to be made constant. Usually, the response speed of the BD sensor when a light beam is incident on the BD sensor changes according to the incident light power. For this reason, if there is variation in the incident light power, on the BD sensor, of the two light beams used for measuring the time interval between the BD signals, variation will appear in the result of measuring the time interval between pulses (BD signals) generated by the BD sensor, and a measurement error can occur. Accordingly, in the case where the time interval between the BD signals is to be measured, the light power of the light beams incident on the BD sensor needs to be made constant by making the light power of the light beams emitted from the light-emitting portions constant.
However, there is a possibility that the light power of the first and second light beams emitted from the two light-emitting portions used in measurement will vary when executing the measurement of the time interval between the first and second BD signals (BD interval) due to an increase in the temperature of the laser driver IC that drives the light-emitting portions. Specifically, if the temperature of the laser driver IC differs significantly between the time of driving the first light-emitting portion that corresponds to the first BD signal, and the time of driving the second light-emitting portion that corresponds to the second BD signal, a variation will occur in the magnitudes of the driving currents supplied to the first and second light-emitting portions. This is because when the temperature of a laser driver IC increases, the driving current output from the laser driver IC decreases due to, for example, an increase in the value of the parasitic resistance in the laser driver IC.
Accordingly, when measuring the BD interval, it is necessary to make the temperature of a laser driver IC as constant as possible at the time of driving the first light-emitting portion that corresponds to the first BD signal, and at the time of driving the second light-emitting portion that corresponds to the second BD signal. In particular, in the case of driving multiple light-emitting portions using multiple laser driver ICs as in Japanese Patent Laid-Open No. 2011-173412, in order to suppress a difference between the driving currents supplied to the first and second light-emitting portions used in measuring the BD interval, driving control of the light-emitting portions needs to be executed as uniformly as possible between the laser driver ICs during driving control other than driving control for causing the light-emitting portions to emit light based on image data.